1. Field of the Invention
The present invention relates to a process for the preparation of phospholipids having modified headgroups and, more particularly, to a process of using phosphatidylhydroxyalkanols as intermediates in the preparation of headgroup modified phospholipids.
2. Description of the Related Art
Phospholipids, such as phosphocholines (polymerizable and synthetic), are used to form technologically attractive, stable lipid membranes, the utility of which is well established in fields such as encapsulation and controlled release, ion-transport, molecular recognition and biosensors. Synthetic alternations in the phospholipid molecules have not only made it possible to fabricate molecularly engineered supramolecular assemblies but also to stabilize the resultant microstructures through polymerization. Phospholipids have been modified, both in the polar headgroup region and in the acyl chain region to meet the specific goal desired for the membrane, including stabilization schemes, site for protein immobilization, controlled release strategies, molecular recognition and sensor development. To extend the applicability of lipid membranes, the charge neutral headgroup of phosphocholines need to be replaced with reactive functionalities which provide sites for further surface modification. The most straightforward routes reported for the synthesis of such headgroup modified lipids have been the phospholipase D mediated transphosphatidylation of phosphocholines with substituted alkanols. Such straightforward routes are not synthetically attractive due to the low yields of products, and the dependence of transphosphatidylation on the reaction conditions, chain length of alkanols as well as the nature of the acyl chains.
For example, in the synthesis of headgroup modified diacetylenic phospholipids by transphosphatidylation, the nature and origin of phospholipase D enzyme plays an important role. Phospholipase D enzyme derived from cabbage exchanges short chain alkanols, but not long chain alkanols, with choline moiety of natural and synthetic phospholipids. Phospholipase D enzyme isolated from rice germ facilitates exchanges of alkanols independent of their chain length. Phospholipase D enzyme extracted from streptomyces catalyzes the transfer of higher alcohols with natural phosphocholines, but remains ineffective on synthetic phosphocholines, mixed chain cholines and bulky choline groups. Similarly, pure phospholipase D enzyme from cabbage or peanut does not produce reproducible yields of diacetylenic phosphatidylethanolamine or phosphatidylbromoethanol.
Alternate synthetic routes involve multi-step, time consuming, low yield chemical synthesis. Furthermore, the reaction conditions involved in these alternate synthetic routes are often incompatible with the polymerizable moieties in lipids which are useful in developing further technological applications.
Thus, there is a need for a convenient, reproducible and high yield general synthetic route for the preparation of a variety of headgroup modified phospholipids.